1. Technical Field
Apparatuses and devices consistent with the present invention relate to light sources for vehicles and the like, and more particularly, to discharge bulbs for vehicles.
2. Description of the Related Art
As a light source of a vehicle headlamp, a discharge bulb equipped with a glass arc tube main body has been used. However, such a discharge bulb has a number of disadvantages. First, a metal halogenide sealed in the glass tube causes corrosion of the glass tube. Second, a proper light distribution cannot be obtained due to the occurrence of a blackening or devitrification phenomenon. Lastly, the life of the discharge bulb equipped with a glass arc tube is not so long. Moreover, a discharge arc chamber of the glass arc tube main body is formed of a glass sphere. Therefore, a sealed material such as the metal halogenide, which is supersaturated, accumulates in a liquid state on the bottom portion in the glass sphere, and a desired light distribution characteristic or a white light distribution color cannot be obtained.
Japanese Patent Application Publication No. JP-A-2004-362978 describes a related art discharge bulb. This related art discharge bulb is shown in FIG. 10. The related art discharge bulb is equipped with a ceramic arc tube main body having a discharge arc chamber in which a pair of discharge electrodes are provided to oppose to each other and a luminous material as well as a starting rare gas is sealed within the discharge arc chamber. More particularly, the arc tube main body has such a structure that both end portions of a circular cylindrical ceramic tube 200, having a thin tube portion to which a pore 201 being in communication with the discharge arc chamber is provided, are sealed by joining a molybdenum pipe 212 to the pores 201 at both end portions of the circular cylindrical ceramic tube 200. Then, a rear end portion of an electrode rod 214, which is inserted into the molybdenum pipe 212 such that a top end portion of the electrode rod 214 protrudes into the discharge arc chamber of the circular cylindrical ceramic tube 200, is joined (i.e., welded) to a rear end portion of the molybdenum pipe 212 that protrudes from the ceramic tube 200. A lead wire 216 is connected to the molybdenum pipe 212 that protrudes from the ceramic tube 200.
Since the ceramic tube 200 is stable for the metal halogenide, the ceramic arc tube main body has a longer lifetime than the glass arc tube main body. Also, the ceramic tube has a higher heat-resistant temperature than the glass tube. Moreover, the end portion of the ceramic tube 200 is formed of a thin tube portion 200b whose inner and outer diameters are smaller than those of a center discharge arc portion 200a. Accordingly, a heat radiation from the arc tube end portion whose surface area is small is reduced and the discharge arc chamber is able to be kept at a high temperature, resulting in increased energy conversion efficiency.
Also, the discharge arc portion 200a of the ceramic tube 200 is shaped into a circular cylindrical shape. When the sealed material such as the metal halogenide, which is supersaturated, accumulates on the lower portion of the discharge arc chamber, the sealed material gathers around a stepped portion 206 of the pore 201 since this is a coolest point in the discharge arc chamber. As a result, the light emitted downward can be utilized effectively and a desired white light distribution can be obtained.
However, the related art discharge bulb shown in FIG. 10 and described in Japanese Patent Application Publication No. JP-A-2004-362978 still has a number of disadvantages. The stepped portion 206 is formed between the discharge arc portion 200a and the thin tube portion 200b at both ends of the discharge arc chamber in the ceramic tube 200. It has been found that when an impact force is generated by dropping the related art discharge bulb, or by contacting the discharge bulb with other objects, a stress is concentrated at the root of the thin tube portion 200b, causing the thin tube portion 200b to bend.
Also, since a thermal stress is applied to the stepped portion 206 due to a temperature difference between the discharge arc chamber and the pores 201 at both ends of the discharge arc chamber, there is a risk that a crack may occur at the root, where the pore opens into the chamber, of the thin tube portion 200b. 
Also, the sealed material such as the metal halogenide, which is supersaturated and accumulated around the lower area of the stepped portion 206 in the discharge arc portion, tends to enter into a minute clearance 215 between the electrode rod 214 and the molybdenum pipe 212 and accumulates there. Therefore, an amount of the metal halogenide that contributes substantially to the discharge arc is reduced, and a luminous efficiency is lowered. Also, a desired luminous flux cannot be maintained over the long term. More specifically, the minute clearance 215 of, for example, about 25 μm is formed between the electrode rod 214 and the molybdenum pipe 212 in the arc tube main body in order to allow the electrode rod 214 to be inserted into the molybdenum pipe 212 during assembly or to absorb a thermal stress generated in the sealing portion at both ends of the ceramic tube 200. However, since the molybdenum pipe 212 and the electrode rod 214 have a good thermal conductivity, a coolest point of the arc tube main body during lighting is located in the inner part of the minute clearance 215 between the electrode rod 214 and the molybdenum pipe 212. This coolest point is thus far from the discharge arc chamber. Accordingly, the metal halogenide which is sealed in the discharge arc chamber is held as a steam or in a liquid or solid state in the inner part of the minute clearance 215 as the coolest point during the lighting of the arc tube main body, and an amount of the metal halogenide that contributes substantially to the discharge arc is reduced correspondingly. As a result, a luminous efficiency is lowered and a desired luminous flux cannot be obtained.
Also, light distribution of the reflector is shaped by pasting radially a light source image of the arc tube main body around cut-off line/elbow portions of the light distribution patterns onto a light distribution screen arranged in front of the lighting equipment. In this case, since an inner diameter of the arc tube main body (i.e., the discharge arc chamber) is large, the light source image also curves in response to the curved arc and thus the cut-off line of the light distribution pattern also waves. In addition, in many cases the sealed material such as the metal halogenide that is supersaturated tends to accumulate on the center bottom portion of the discharge arc chamber. Therefore, a brightness difference in the pasted light source images is revealed as unevenness of the light distribution in the light distribution pattern since the brightness in the center bottom portion of the discharge arc chamber is low, and thus a proper white light distribution cannot be obtained.
To account for this unevenness, the linear white light source image must be formed by shielding the emergent light to the side or lower portion of the discharge arc chamber (i.e., by shielding the lower half of the arc tube main body in the circumferential direction). Thus, a conversion efficiency into the effective luminous flux decreases since the emergent light is shielded.